A Cladistic Analysis of Pseudocoelomate (Aschelminth) Morphology

Abstract

We investigated possible relationships among pseudocoelomates (aschelminths) by using cladistic analysis to uncover all most-parsimonious trees from a data set comprising 45 morphological characters of 11 phyla (3 acoelomate, 7 pseudocoelomate, and 1 coelomate). Analysis of our matrix yielded 2 most-parsimonious trees, differing only in relative placement of 3 taxa (Kinorhyncha, Loricifera, and Priapulida). Tree topology suggests that the pseudocoel either evolved twice, or that it evolved once and was then independently lost in Gastrotricha and in Gnathostomulida. Tree topology also is consistent with derivation of the pseudocoel from a coelom. From our analysis and that of other morphologically-based studies, we conclude that there are 2 main lines of pseudocoelomate evolution: (1) a minor clade (Acanthocephala + Rotifera) defined principally by a syncytial epidermis, intracytoplasmic lamina, and sperms with anterior flagella; (2) a major clade (the 5 remaining phyla), defined primarily by a cuticle that is molted. Within the major clade are 2 subclades (Nematoda + Nematomorpha; Kinorhyncha + Loricifera + Priapulida). Using morphological data, the chief issues remaining to be resolved are placement of Gastrotricha and Gnathostomulida, and evolution of the pseudocoel. No analysis based solely on morphological data has so far produced a tree congruent with those based on molecular data. Nevertheless, there is a general consensus that (1) the pseudocoel is of little phylogenetic significance and (2) the pseudocoelomates are probably polyphyletic. Additional key words: evolution, phylogeny the face of this bewildering array of conflicting opinions about the interrelationships of the aschelminth phyla, it is impossible to form a coherent picture of the evolution of the animals. (Clark 1964) Does Clark's observation made some 30 years ago still offer a realistic view of our understanding of pseudocoelomate (aschelminth) phylogeny? Clearly, these vermiform taxa share interesting similarities. However, they also possess some striking differences, which have led many authors to choose not to include them in a single phylum, the Aschelminthes (e.g., Clark 1964; Anderson 1983; Inglis 1985; Pearse et al. 1987; Brusca & Brusca 1990; Kozloff 1990; Willmer 1990; Ruppert & Barnes 1994). In fact, for Inglis (1985) there is no common history at all, just completely independent solutions to similar evolutionary pressures. Further complicating the matter is Ruppert's (1991) view that calls into question the significance of 2 traditional characters that link the pseudocoelomates: pseudocoel and cuticle (see also Brusca & Brusca 1990). The alternative position, of course, argues for at least a loose relationship among some of these taxa (e.g., Hyman 1951; Boaden 1985; Lorenzen 1985; Sterrer et al. 1985; Bergstrom 1991; Meglitsch & Schram 1991; Malakhov 1994). The problem is to dissect possible relationships from what appears to be an intractable array of characteristics. However, there is no reason to abandon the logical search for phylogenetic relationships among these taxa. In fact, many workers have explored this field, with phylogenetic analyses that include most or all of the appropriate taxa (e.g., Lorenzen 1985; Meglitsch & Schram 1991; Schram 1991; Eernisse et al. 1992; Conway Morris 1994; Nielsen 1994; 1995; Neuhaus 1994; Winnepenninckx et al. 1995). Here we report our findings from a cladistic analysis of 45 morphological characters and 11 taxa (3 acoelomate, 7 pseudocoelomate, and 1 coelomate). We also compared trees yielded from this study to other published works. Thus, the purposes of this study were to assemble a useful set of light microscopical, ultrastructural, and biochemical characters on the pseudocoelomate taxa and to develop parsimonious cladograms (hypotheses) from those data. Such work may provide a useful foundation for the molecular data that are now forthcoming (e.g., Lake 1990; Brandl et al. 1992; This content downloaded from 157.55.39.30 on Wed, 20 Jul 2016 06:18:46 UTC All use subject to http://about.jstor.org/terms Phylogeny of pseudocoelomates Smothers et al. 1994; Garey et al. 1995; Winnepenninckx et al. 1995). A combination of morphological and molecular data may eventually lead to a comprehensive phylogeny of these taxa (cf. Hillis 1987; Patterson et al. 1993).